The present disclosure relates to a surface emitting device suitable for use as a direct-lighting backlight unit of a liquid crystal display or the like and a liquid crystal display including the same.
Liquid crystal displays having a low-profile and easily viewable backlight (surface emitting device) have been used as displays of word processors, laptop personal computers and so on. To meet the need for such lightweight and low-profile backlights for liquid crystal displays, edge-lighting type backlight units have been mainstream in which a line light source such as a fluorescent tube is disposed at the side end of a transparent plate (light guide), and a liquid-crystal display panel is disposed on the light guide with a light diffusing sheet or the like in between.
However, such edge-lighting backlight units often become lacking in luminance as displays are becoming larger in size in recent years. Therefore, direct-lighting type backlight units come into use in which a line light source and a light diffusing sheet are disposed directly under a liquid crystal panel.
FIG. 13 is a schematic perspective view of a conventional direct-lighting type backlight unit 1. The direct-lighting type backlight unit 1 includes line light sources 2 such as fluorescent tubes, a reflector 3, a diffuser 4 and a diffusion sheet 5 serving as light diffusing sheet. The reflector 3 is disposed under the line light sources 2. The diffuser 4 is disposed above the line light sources 2. The diffusion sheet 5 is disposed on the diffuser 4. The diffuser 4 and the diffusion sheet 5 form the light-exiting surface of the backlight unit.
In this backlight unit 1, the light emitted from the line light sources 2 exit from the diffuser 4 and the diffusion sheet 5. The luminance of the light of the backlight unit 1 is high directly above the line light sources 2 and low above the portions between the light sources 2, as shown in FIG. 14(A).
Therefore, it is known to provide a prism sheet 6 between the line light sources 2 and the diffusion sheet 5 so as to divide the light-source image (emission line), as shown in FIG. 14(B) (refer to Japanese Unexamined Patent Application Publication No. 5-333333, Japanese Unexamined Patent Application Publication No. 6-250178, and Japanese Unexamined Patent Application Publication No. 10-283818). FIG. 14(B) shows an example in which a prism sheet 6 is used as a light-source image dividing sheet in place of the diffuser 4 in FIG. 14(A). The prism sheet 6 is an optical film which is generally used as a luminance raising film, and has a plurality of triangular-cross-section linear protrusions (prisms) 6a disposed continuously at regular pitches on the front or back surface.
The prism sheet 6 is disposed so that the edge lines of the linear protrusions 6a are in parallel to the extending direction of the line light sources 2. This doubles the light-source image of the light exiting from the diffusion sheet 5 in comparison with that without the prism sheet (FIG. 14(A)), thereby distributing the luminance uniformly.
The above-described method of dividing the light-source image using the prism sheet 6 needs to divide the light-source image into two main images at a short interval so as to distribute the luminance uniformly in an efficient manner.
FIG. 15 is an optical path diagram showing the relationship between the prism sheet 6 and the line light source 2. Light L1 emitted from the line light source 2 in the direction of the normal to the prism sheet 6 is totally reflected by the slopes (prism surfaces) of the linear protrusions 6a of the prism sheet 6 to return to the line light source 2, and is again used as light incident on the prism sheet 6. Light L2 that enters the prism sheet 6 at an angle is raised to the front surface (in the direction of the normal) by the linear protrusions 6a, and exits therefrom to form light-source images LA. Light L3 that enters the prism sheet 6 at a larger angle is reflected by the bottom surface of the prism sheet 6 to be returned to the line light source 2. Accordingly, the light from one line light source 2 is divided into two light-source images LA at the front surface of the prism sheet 6.
In short, the conventional backlight unit 1 using the light-source image dividing sheet 6 can divide the light into only two main light-source images, which is not enough to distribute the luminance uniformly.